ABSTRACT. The ingestion of invertebrates by herbivores on rocky intertidal shores is traditionally considered a casual phenomenon. However, a recent study of 29 species in northern Chile shows that animal consumption is widespread, consistent, and important, suggesting that some of these herbivores may actually be omnivores. Therefore, we examined the capability of three common Chilean herbivores (the key-hole limpets Fissurella limbata and Fissurella pida and the polyplacophoran Chiton granosus) to digest animal food. For each species, we conducted no-choice feeding experiments using artificial foods based on either algal or animal tissue from one of their frequent prey (Ulva rigida, Perumytilus purpuratus). After the feeding trials, we evaluated the total proteolytic activity (availability of free amino acids) in the digestive contents of the species studied and, as a reference, we evaluated this activity in animals obtained directly from the field. We found that all three species were able to eat animal food, and this consumption was not significantly different from that of algal food, suggesting that both foods were not only edible but at least similarly palatable. In addition, we detected comparable levels of proteolytic activity under the three feeding conditions for the three species. No statistical differences were found for C. granosus, but activity was significantly higher with animal food in F. limbata and with algal food in F. pida. Our data show the high digestive flexibility of these species, suggesting their ability for adaptive modulation and the possibility that they are true omnivorous consumers. We discuss the implications of these results for our current view of the structure of rocky intertidal food webs.

Limpets, key-hole limpets, placophorans, and snails are common constituents of invertebrate assemblages on Chilean rocky intertidal shores. These species are traditionally considered to be herbivorous consumers that play a key role in the structure and organization of littoral communities (Moreno & Jaramillo, 1983; Santelices, 1989; Fernández & Castilla, 2005). Although the diet of these species is long recognized to include an important proportion of invertebrate animals (Santelices & Correa, 1985; Osorio et al, 1988), this phenomenon has been largely interpreted as casual or incidental and, thus, the species are usually referred to as primarily herbivorous (López et al, 2003).

However, a recent trophic evaluation of 29 herbivore species in northern Chile, conducted in four communities over 1.5 years, showed that their ingestión of animal items was common and widespread (Camus et al, 2008). These herbivores consumed, on average (mean ± s.e.), a total of 48.2 ±5.1 items per species, of which 26.2 ± 1.4% were animal items. This proportion was positively related to herbivore body mass and exceeded 60% for particular species at given times and places (e.g., Fissurella species, up to 82% in F. limbata). In addition, eight of these species ingested heterospecific herbivores that were also potential competitors, thus making such interactions equivalent to the more complex phenomenon of intraguild predation (see Polis et al., 1989) and turning the herbivore trophic level into a sub food web with trophic chains up to three levels in length (see analysis in Camus et al., 2008).

The above results strongly suggest that some of these herbivores could actually be omnivores that use animals as a nutritional source. Such a possibility is far firom remote, as recent analyses have demonstrated trae omnivory and even carnivory in chiton and fissurellid species from other coasts (Latyshev et al., 2004; Hughes et al., 2005; Grall et al., 2006). In addition, the existence of omnivory may have numerous implications for the understanding of food web stractures and dynamics on rocky shores (Navarrete et al., 2008; Sanhueza et al., 2008), and should not be ne-glected in experimental studies of community organization and regulation. In fact, herbivores were long proposed to be termed predators, as they play an equivalent functional role to carnivores in terms of predator-prey dynamics (Lubchenco, 1978); this suggestion should make more sense if these herbivores indeed turn out to be omnivores. However, the above implications have not been explored so far, and we suggest that the main reason is that we have always treated and referred to these intertidal consumers as herbivores.

In this context, we conducted a non-choice experimental assessment of the capability of rocky intertidal herbivores to consume and digest algal versus animal food, selecting three common grazing mollusks widely distributed on the Chilean coast (from 18o to at least 42°S; Oliva & Castilla, 1986; Hidalgo et al., 2006): the polyplacophoran Chiton granosus Frembly, and the key-hole limpets Fissurella limbata Sowerby and Fissurella picta (Gmelin). These species have a strong potential to be physiological omnivores (Camus et al, 2008) and, therefore, we focused on the detection of specific proteolytic activity (such as the avail-ability of free amino acids) within their digestive tract after ingesting animal food. Such findings have been suggested to offer primary evidence for omnivory (e.g., Clark et al, 1985; Sabapathy & Teo, 1995), although they constitute a necessary albeit insufficient condition for the adequate digestión of animal prey. Some trae herbivores exhibit high proteolytic activity when processing algal food (Horn, 1989; Ojeda & Cáceres, 1995; Johnston & Freeman, 2005), so we expect them to exhibit noticeably lower activity when feeding exclusively on animal food. For example, herbivorous fishes may have intensive proteolytic digestion because their long digestive tracts allow a longer exposure of food to enzymatic action, although the proteolytic activity per volume unit of gut content is clearly lower in herbivores than in omnivores or carnivores (Hofer & Schiemer, 1981). For our study species, thus, the detection of comparable proteolytic activity for algal and animal food would be a first suggestion of an omnivorous habit, provided that animal preys are indeed a regular component of their diet. Additionally, such results may be better understood in the frame of the adaptive modulation hypothesis. In this context, omnivores are expected to have the capability to modulate enzymatic activity and nutrient transport depending on the dietary substratum; this cannot be done by specialized animals such as strict herbivores (e.g., Buddington et al, 1987; Sabat et al, 1998). Therefore, the capability of a given herbivore to display at least similar levels of proteolytic activity when consuming algal versus animal food will be herein taken as evidence of digestive flexibility and preliminary support for the hypothesis that the species is omnivorous.

MATERIAL AND METHODS

Apparently, no published information exists on the particular characteristics of digestión in C. granosus, F. limbata, or F. picta. However, the three species have similar and structurally simple digestive systems, in accordance with the generalized model of mollusks (Ruppert & Barnes, 1994); the main components are the esophagus, stomach, and intestine. Buccal salivary glands secrete mucus to facilitate the transport of food into the esophagus, where esophageal (pharyngeal) glands reléase enzymatic secretions for carbohydrate digestión. Proteolytic activity is largely extracellular and occurs in the stomach, where the food mixes with enzymatic secretions released into the lumen by digestive glands (Fretter & Graham, 1994; Ruppert & Barnes, 1994; Sasaki, 1998). Although some digestión may occur in the anterior intestine of chitons, the main role of the intestine is the compaction of feces. The rate of food passage through the digestive tract depends, to a large extent, on the intestinal length, which can vary between closely related species. As some Fissurella species firom Venezuela exhibit food passage rates of 6 to 12 h (Franz, 1990), we used this range as a reference to establish the acclimation periods and the duration of feeding triáis in our experiments.

In winter and spring 2007, individuals of C. granosus, F. limbata, and F. picta were collected firom the intertidal zone of Caleta Cocholgüe (36°35'S), on the coast of Concepción, Chile, and then transported to the laboratory in containers with seawater and aera-tion. We then conducted parallel evaluations of the three species following the same protocol. For each species, the collected individuals were separated into three experimental groups of six individuals each (seven in the case of C. granosus). The first group of individuals was sacrificed immediately after collection to obtain a reference level of the proteolytic activity of animals feeding on a field diet. Prior to the experi-ments, the remaining individuals were maintained in aquaria (separated by species) with circulating sea-water at 14°C and constant aeration for three days, without food. Such conditions allowed the acclimation of individuals for subsequent experiments, and the complete elimination of contents from their digestive tract. After acclimation, animals were separated into two groups in different aquaria, placed in individual chambers, and fed for six days with artificial food (either animal or algal tissue) (10%) homogenized in an agar matrix (1%) and diluted in filtered seawater (see description below). Each day of the experiment, individual animals were left for 5 h in a Petri dish covered with a layer of 5 g of food; this was done to measure the amount of food consumed each day. These results are referred to as daily consumption, although we note that they correspond to a 5-h period.

Artificial foods were prepared with powdered tissue of the mussel Perumytilus purpuratus (Lamarck) and the green alga Ulva rigida C. Agardh, hereafter referred to as animal and algal food, respectively. These two species are among the most common organisms on Chilean rocky intertidal shores (Camus & Andrade, 1999; Prado & Castilla, 2006), and both of them are usually ingested in the field by the three herbivores studied (Camus, unpublished results). The protein content of U. rigida is, on average, approximately 18% (dry weight), showing temporal variations between ca. 11 and 24% (Rouxel et al, 2001). We did not find equivalent information for P. purpuratus in the literatee, although a recent estimation indicates a protein content of nearly 55% (Figueroa & Fariña, unpublished data), similar to the amount reported for other mussel species (e.g., Mytilus trossulus Gould, with an average of ca. 50%, ranging from ca. 44 to 57%; Kreeger, 1993).

At the end of feeding experiments, individuals were dissected and their digestive tract (from esophagus to anus) was removed to obtain its content (food along with enzymes secreted into the lumen) and measure its total proteolytic activity. The digestive content was centrifuged (13,000 rpm for 20 min), the supernatant was blended (2:1) with a solution of cold acetone (-20°C), and then it was centrifuged again (13,000 rpm for 12 min). Later, 1.0 mg of the digestive extract was diluted in 10 mL phosphate buffer (pH 7); 1 mL of this solution was added to 2 mL bovine serum albumin (BSA) and incubated for 1 hour at 37°C. Proteolytic activity was assessed in terms of amino acid released and using the ninhydrin method (Troll & Cannan, 1953); it was measured at 570 nm in a spectrophotometer and compared against an 1-alanine calibration curve. The results were expressed as mg alanine mL-1h-1 . Both food consumption and proteolytic activity were evaluated separately for each species by means of one-way ANO VA, excepting one case that did not meet the assumptions and was evaluated with a Kruskal-Wallis test (Siegel & Castellan 1988).

RESULTS

Food consumption

As expected, the three herbivore species were all able to consume algal food, but our results also showed that they had no impediment to consume animal food. Figure 1 shows the mean daily consumption rate, which, on average, varied from 0.24 (F. limbata) to 0.418 (C. granosus) g day-1 of algal food, and from 0.21 (F. picta) to 0.49 (C. granosus) g day-1 of animal food. The consumption rates did not differ significantly for C. granosus (F1,12 = 4.60, p = 0.053), F. limbata (F1,10 = 0.177, p = 0.683), or F. picta (F1,10 = 1.656, p = 0.227). Although these results might be partly attributed to the effect of starvation, no individual showed signs of avoiding the animal food, and the lack of difference in the consumption of both foods indicates that they had a similar edibility for the herbivores.

Proteolytic activity

The digestive tract of the three species exhibited proteolytic activity under the three conditions examined (field, animal, and algal diets). The average enzymatic activity in C. granosus (Fig. 2a) ranged from 0.13 (both for field and animal diet) to 0.16 (algal diet) mg alanine mL-1 h-1 . In spite of the higher activity with algal food, no significant differences were found among the three conditions (Kruskall Wallis; H = 0.636, p = 0.728, n = 18). This result was particularly important because it suggests that the consumption of animal food by C. granosus does not convey a reduc-tion of its digestive capability.

Somewhat different results were obtained for the Fissurella species. F. limbata (Fig. 2b) showed a higher variation in proteolytic activity ranging from 0.12 (algal) to 0.17 (animal) mg alanine mL-1 h-1 , with significant differences between the three conditions (F1,19 = 9.70, p = 0.0013). A posteriori comparisons revealed significantly higher activity for animal than for algal food (Tukey; p = 0.00095), without differences among the remaining comparisons (0.082 ≤ p ≤ 0.114). In contrast, proteolytic activity in F. pida (Fig. 2c) ranged from 0.12 (field) to 0.16 (algal) mg alanine mL-1 h-1 , with significant differences between the three conditions (F1,19 = 9.03, p = 0.0018), although with significantly higher activity for algal food as compared with animal food (Tukey; p = 0.0051) and the field diet (p = 0.0071), but with no difference between these latter two (p = 0.987). As with C. granosus, the proteolytic activity of the two Fissurella species fed with animal food was never lower than with the other diets.

DISCUSSION

Our results are, to the best of our knowledge, the first data suggesting the existence of trae (physiological) omnivory in Chilean invertebrate consumers previously typified as herbivorous, and we give particular importance to the consistent response of the three species. The experiments did not test for food prefer-ence but, based on the natural diet of these species (Camus et al, 2008), we expected them to consume a greater amount of algal than animal food. Therefore, the results also suggest that both algal and animal food were not only edible but at least similarly palatable for the three species, even for C. granosus, which con-sumed nearly twice as much food as F. limbata and F. pida. The variable levels of proteolytic activity exhibited by individuals that consumed a natural diet were expected because they are unlikely to eat exactly the same resources in the field. Thus, it is important to note that such levels were always comparable to those recorded with artificial foods, suggesting that our data closely reflect what animals do in nature.

The levels of proteolytic activity recorded for experimental or field diets ranged from 0.12 to 0.17 mg alanine mL-1 h-1 , and are difficult to compare with published data because of the variety of units and calibrations used by different authors. As a reference, thus, we converted the data on total protease activity (µg tyrosine min-1) obtained by Johnson & Freeman (2005) for six species of crab with varyied feeding habits into comparable units (mg h-1 ). The transformed valúes for crab species fluctuated between a minimum of 0.03 mg h-1 for the herbivorous Nedocarcinus in tegrifrons (Latreille) and máxima of 0.059 mg h-1 for the carnivorous Plagusia chabrus (Linnaeus) and 0.072 mg h-1 for the omnivorous Leptograsus variegatus (Fabricius). Johnson & Freeman (2005) suggested that the higher total activity in L. variegatus with respect to P. chabrus was due to the ingestion (likely incidental) of some red algae by the latter species, which nonetheless showed the highest trypsin activity (ca. 1.5 times that of L. variegatus), in accordance with its carnivorous status. However, our results are not directly comparable because Johnston & Freeman (2005) used ahomogenate of the digestive glands after crab species had consumed their natural diet in the field, whereas we used gastric contents and our species were experimentally fed with a single type of food. In this regard, we note that our three study species displayed high proteolytic activity under all feeding conditions (including field diet) and, in spite of the statistical differences detected in Fissurella species, they all evidenced high digestive flexibility and the capability to display adaptive modulation (Buddington et al, 1987; Sabat et al, 1998). Although we failed to test a third experimental food (mixing algal and animal tissue), we predict that combining the high protein content of both foods should result in a higher proteolytic activity than that observed for each food separately, as expected in the case of omnivorous species. Nonetheless, we note that our results are not a demonstration of the many aspects and implications of omnivory, and they should also be verified employing more accurate techniques (e.g., see Latyshev et al, 2004). Notwithstanding, this study highlights the po-tential of the three studied species to switch between animal and algal foods, an ability that allows omni-vores to take advantage of a wide variety of resources in the field and exploit nutritionally unpredictable environments (e.g., Moyle et al, 1995).

Therefore, we suggest that similar analyses should be conducted for other intertidal consumers, particularly mollusk and echinoderm species, considering that the most important Chilean herbivores are also strong candidates to be omnivores (Camus et al, 2008). An evaluation of the existence and prevalence of omnivory may lead to drastic changes in our perception of the functional role of herbivore consumers both from ecophysiological and predator-prey perspectives, with at least two immediate consequences. Once a given species is demonstrated to be omnivorous, a logical step is to assess the relative importance of algal versus animal food for its nutritional and energetic balance, and to relocate its trophic position within the community food web. On the other hand, the evaluation of real food webs often relies on the assignation of the omnivorous status based on species diets; without acknowledging the distinction between eating and feeding. Some commonly used concepts of omnivory refer to feeding at more than one trophic level (Pimm & Lawton, 1978), being linked to a basal species at more than one link in a trophic chain (Yodzis, 1984), or eating basal (plant) and intermediate (animal) species (Arim & Marquet, 2004). In the case of herbivores, however, the incidental consumption of animals is indeed a common phenomenon and, therefore, omnivory should not be readily assumed firom dietary composition.

In a different context, the consumption of animals by herbivores, even if incidental, may be important as a potential source of indirect effects on their prey (Camus et al, 2008). However, it may turn out to be especially significant if trae omnivory is involved, particularly for Chilean rocky shores where omnivores are usually assumed to be absent. In this context, the eventual demonstration of omnivory should forcé a reappraisal of the structure, dynamics, and stability of rocky intertidal food webs (Navarrete et al, 2008; Sanhueza et al., 2008), particularly with regard to top-down and bottom-up cascading effects. Actually, Chilean intertidal herbivores are highly connected species contributing a high number of weak trophic links (Camus et al, 2008). Thus, changing their status firom herbivorous to omnivorous would not only involve the existence of longer food chains but also the addition of many omnivore links (Yodzis, 1984), closed omnivore loops (Sprales & Bowerman, 1988), and a higher incidence of intraguild predation with potential importance for food web dynamics (see Arim & Marquet, 2004). The fact that such a reappraisal can be triggered firom a simple assessment of the capacity to digest animal food should, thus, be considered an incentive for conducting research in that direction.

ACKNOWLEDGEMENTS

We thank financial support firom CONICYT, Chile (project FONDECYT 1040425), and the criticisms by an anonymous reviewer which were extremely helpful for improving the context and ideas of the manuscript.